Speaking at a location just outside the South Korean capital, Prof Kim told the BBC he has regular contact with key figures within the country who have intimate knowledge of the military's cyber operation.

"The size of the cyber-attack agency has increased significantly, and now has approximately 6,000 people," he said.

He estimated that between 10% to 20% of the regime's military budget is being spent on online operations.

"The reason North Korea has been harassing other countries is to demonstrate that North Korea has cyber war capacity," he added.

"Their cyber-attacks could have similar impacts as military attacks, killing people and destroying cities."

Stuxnet clone

Speaking more specifically, Prof Kim said North Korea was building its own malware based on Stuxnet - a hack attack, widely attributed to the US and Israel, which struck Iranian nuclear centrifuges before being discovered in 2010.

"[A Stuxnet-style attack] designed to destroy a city has been prepared by North Korea and is a feasible threat," Prof Kim said.

"Although the nuclear plant was not compromised by the attack, if the computer system controlling the nuclear reactor was compromised, the consequences could be unimaginably severe and cause extensive casualties," Prof Kim said.

Martyn Williams is a journalist who follows closely the development of technology in North Korea.

He told the BBC: "I think it's important to underline that this is theoretical and possible from non-North Korean hackers too.

"It's conceivable that hackers would try something and lives could be at risk.

He noted an attack in 2013 on South Korean broadcasters, which he said was "an attempt to throw the country into confusion".

"If TV had gone off air and then ATMs stopped working, people might have panicked."

Inside Bureau 121

When it comes to cyber-attacks, few groups are as notorious as North Korea's Bureau 121, which has operated since the late nineties.

Most security researchers agree that the group operates out of China. Specifically, in the basement of a restaurant, rated highly on TripAdvisor for its tremendous Korean food.